1. Field of the Invention
This invention relates to transparent plastic sheets and, more particularly, to transparent plastic sheets for use as aircraft windshields.
2. Description of the Prior Art
Methacrylic resins are being used in a wide variety of industrial fields owing to their characteristic features including excellent optical properties, good weather resistance, and feeling of high quality. At present, the range of their use shows a tendency to increase still more. In particular, their use as aircraft windshields has been an important field of application since prewar days.
Although MIL standards have been established as standards for aircraft windshields and drawn sheets for use as aircraft windshields, their performance requirements are becoming gradually severer with the development of aircraft.
Consequently, MIL-P-8184E including an alteration of the standard for craze resistance and the establishment of a standard for long-term water absorption percentage has been set up.
Craze resistance is one of the very important properties of aircraft windshields. In order to improve this property, it has been common practice to crosslink the resin material. However, an increased degree of crosslinking may make it difficult to form the resin material into windshields. Moreover, in order to improve craze resistance at a low degree of crosslinking, there has been proposed a method in which methyl methacrylate is copolymerized with neopentylene glycol dimethacrylate (Japanese Patent Laid-Open No. 192414/'87).
Although this method is effective in improving craze resistance, wavy sink marks are produced on the surface of the resulting sheet. In order to use this sheet as a windshield, its surface must be smoothed by grinding or the like.
Alternatively, the copolymerization of methyl methacrylate with methacrylamide, which is practically used to make aircraft windshields, can also produce an improvement in craze resistance. However, the resulting sheet has the problem of being tinged with yellow.
Thus, the prior art relating to aircraft windshields can bring about an improvement in craze resistance. However, the resulting sheets present such problems as coloring, sink marks on the surface, and an increase in long-term water absorption percentage, and cannot be regarded as satisfactory.